Deposition technologies are typically defined as technologies that deposit functional materials dissolved and/or dispersed in a fluid onto a receiver (also commonly known as substrate etc.). Technologies that use supercritical fluid solvents to create thin films are known. For example, R. D. Smith in U.S. Pat. Nos. 4,582,731, 4,734,227 and 4,743,451 discloses a method involving dissolution of a solid material into a supercritical fluid solution and then rapidly expanding the solution through a short orifice into a region of relatively low pressure to produce a molecular spray. This may be directed against a substrate to deposit a solid thin film thereon, or discharged into a collection chamber to collect a fine powder. By choosing appropriate geometry of the orifice, and maintenance of temperature, the method also allows making of ultra-thin fibers from polymers. This method is known as RESS (rapid expansion of supercritical solutions) in the art.
In general, a process is considered a RESS process when the functional material is dissolved or dispersed in a supercritical fluid or a mixture of supercritical fluid and a liquid solvent, or a mixture of a supercritical fluid and surfactant, or a combination of these, which is then rapidly expanded to cause simultaneous precipitation of the functional material. Tom, J. W. and Debenedetti, P. B. discuss RESS techniques in “Particle Formation with Supercritical Fluids—a Review,” J. Aerosol. Sci. (1991) 22:555–584, and also their applications to inorganic, organic, pharmaceutical and polymeric materials. The RESS technique is useful to precipitate small particles of shock-sensitive solids, to produce intimate mixtures of amorphous materials, to form polymeric micro-spheres, and deposit thin films. One problem with RESS based thin film deposition technologies is that it is limited only to materials that are soluble in supercritical fluid. While it is known that co-solvents can improve the solubility of some materials, the class of materials that can be processed with RESS based thin film technologies is small. Another significant problem is that such technologies fundamentally rely on formation of functional material particles through sudden reduction of local pressure in the delivery system. While the reduced pressure reduces the solvent power of the supercritical fluid, and causes precipitation of the solute as fine particles, the control of the highly dynamic operative processes is inherently very difficult. When co-solvents are used in RESS, great care is required to prevent dissolution of the particles by condensing solvent in the nozzle or premature precipitation of particles and clogging in the nozzle. Helfgen et al., in “Simulation of particle formation during the rapid expansion of supercritical solutions”, J. of Aerosol Science, 32, 295–319 (2001), discuss how the nucleation of particles upon supersonic free-jet expansion, and subsequent growth by coagulation at and beyond Mach disk, pose significant design challenges in controlling the particle characteristics. In addition, beyond the expansion device, the complex transonic flow of gaseous material must be managed such that the particles are deposited onto a surface and do not remain suspended in the expanded gas. This is dependent not only on fluid velocities but also on particle characteristics. A third problem pertains to the use of RESS methods in manufacturing: it is well recognized that progress to a fully continuous RESS process is limited by depletion of the stock solution to be expanded. Thus, there is a need for a technology that permits improved control of particle characteristics so that uniform thin films onto receiver surfaces can be deposited continuously with compressed carrier fluids for a broader class of materials.
Fulton et al. in “Thin fluropolymer films and nanoparticle coatings from the rapid expansion of supercritical carbon dioxide solutions with electrostatic collection”, Polymer, 44, 3627–3632 (2003), describe a process that charges the homogeneously nucleated particle as they are formed with an electric field applied to the tip of the expansion nozzle. The charged particles are then forced to a solid surface in this field generating a uniform particle coating. This method, however, does not overcome the limitations of the RESS process, namely, control of particle characteristics, and its limited applicability to only materials soluble in supercritical fluid or its co-solvent mixture.
Sievers et al. U.S. Pat. No. 4,970,093 disclose a process for depositing a film on a substrate by rapidly releasing the pressure of a supercritical reaction mixture to form a vapor or aerosol that is not supercritical. A chemical reaction is induced in the vapor or aerosol so that a film of the desired material resulting from the chemical reaction is deposited on the substrate surface. Alternatively, the supercritical fluid contains a dissolved first reagent, which is contacted with a gas containing a second reagent, which reacts with the first reagent to form particles of the desired material deposited as a film on the substrate. In either case, the method still relies on particle formation upon expansion and suffers from the limited control of particle characteristics and only a narrow class of materials are suitable for processing by this method.
Hunt et al. U.S. 2002/0015797 A1 describe a method for chemical vapor deposition using a very fine atomization or vaporization of a reagent containing liquid or liquid-like fluid near its supercritical temperature by releasing it into a region of lower pressure, where the resulting atomized or vaporized solution is entered into a flame or a plasma torch, and a powder is formed or a coating is deposited onto a substrate. In this particular RESS process, rapid depressurization of a supercritical fluid creates an aerosol of liquid droplets. While further extending the number of possible usable precursors, this method does not improve the prior art in terms of particle characteristic control as particle nucleation and growth processes interact with the energetic regions of the combustion flame or plasma in an uncontrolled fashion.
Sievers et al. U.S. Pat. No. 5,639,441 describe an alternative RESS process and apparatus for forming fine particles of a desired substance upon expansion of a pressurized fluid, wherein the substance is first dissolved or suspended in a first fluid that is immiscible with the second fluid, which is then mixed with the second fluid that is preferably in its supercritical state, and the immiscible mixture is then reduced in pressure to form a gas-borne dispersion of liquid droplets. The method thus relies on atomization and coalescence of fluid droplets rather than nucleation and growth of particles in the supercritical fluid. It is essentially a RESS process as it seeks to make liquid particles through rapid expansion of supercritical fluids. The dispersion then is dried or heated to facilitate reactions to occur at or near surfaces to form coatings or fine particles. The particle formation in this process occurs well beyond the expansion region and occurs through mechanisms similar to those operative during conventional spray or film drying.
U.S. Pat. No. 4,737,384 to Murthy et al. describes a process for depositing a thin metal or polymer coating on a substrate by exposing the substrate at supercritical temperatures and pressures to a solution containing the metal or polymer in a solvent and reducing the pressure or temperature to sub-critical values to deposit a thin coating of the metal or polymer on the substrate. Since the process relies on particle and film formation upon the expansion of the supercritical solution, it is still a RESS process.
U.S. Pat. Nos. 4,923,720 and 6,221,435 disclose liquid coatings application process and apparatus in which supercritical fluids are used to reduce, to application consistency, viscous coatings compositions to allow for their application as liquid spray. The method comprises of a closed system and relies on decompressive atomization of liquid spray for the formation of a liquid coating. Once again, the method is a RESS process as it depends on rapid expansion of supercritical fluids to form liquid droplets.
U.S. Pat. No. 6,575,721 discloses system for continuous processing of powder coating compositions in which supercritical fluids are used to reduce, to application consistency, viscous coatings compositions to allow for their application at a lower temperature. While the method comprises of continuous processing, it still relies on rapid expansion of supercritical fluids to form liquid droplets that are spray dried, and thus, is a RESS process.
Thus, there is still a strong need for a compressed fluid based coating process that operates continuously, has improved control of particle formation for a broader class of materials than hitherto possible with RESS based processes, and can be used to coat these particles onto a substrate uniformly.